In general, optical recording media offer a number of advantages over conventional magnetic tapes or disks in that the former have the ability to store binary data at a high recording density and good reliability for an extended period of service time. Examples of such optical recording media include a magneto-optical disk, a phase-change disk, a write-once read many memory and a compact disk all being designed to store data/information in an optically readable condition. Among them, a magneto-optical disk and a phase-change disk are characterized by the added advantages of data erasability and rewritability, which make them more attractive than the other types of optical recording media.
As well known in the art, digital data is written on a recording layer of such rewritable optical disks by the radiation of a laser beam. The magneto-optical disk records data with heat generated by the laser beam and magnetization characteristics of a magnetic recording layer thereon. On the other hand, the phase-change disk records data using a phase-change recording technique wherein the laser beam is intensity modulated so that a crystallized recording layer thereof goes through a phase change between a crystalline phase and an amorphous phase. Such modulation is normally done by a modulation system generally called an EFM (eight-to-fourteen modulation) system to modulate the laser beam so that a data modulated signal representing digital data, e.g., an EFM signal, is recorded on the optical disk by the modulated laser beam.
However, in the phase-change recording technique, as shown in FIG. 3, tear drops 76 due to thermal diffusion by the Laser beam may occur on a crystallized recording area 72 of a phase-change disk 70, which tends to increase the recording error rate.
In order to overcome the above disadvantage, efforts have been made to find ways of reducing the laser power requirement to prevent the thermal diffusion over the recording area. However, in this case, an recording area having a shortest bit length, e.g., 3T, where T represents a clock pulse period, may be made unstable due to the failure to provide to that area a required level of laser power.